The present invention relates to a method for transmitting signaling and control information for wavelength-division multiplex (WDM) networks for use in optical, fiber-bound information transfer in digitized form.
Various methods are known from the field of transmission technology for transmitting a plurality of independent signals in order to transfer data, speech, text or images over a common medium. This can be achieved in that the useful information underlying the signals differs with regard to its timing slot, frequency or coding. Frequently, it is necessary for so-called signaling and control information (overhead information) to be transmitted in addition to the useful information. Such overhead information can be used to control the participating network elements, to identify the transmitted signals or to determine the transmission quality. Using conventional digital transmission methods, such as PDH and SDH time-division multiplex methods, overhead information can be easily transmitted in addition to the useful information by employing time slots reserved for this purpose. Methods of this kind can be implemented at very low cost using electronic digital technology.
In optical transmission technology, it is known for a plurality of signals to be transmitted via a common medium, the optical fiber, in that, prior to optical modulation, a plurality of signals are multiplexed using conventional transmission technology means . The disadvantage of this method is that the individual signals or the overhead information that is contained cannot be accessed using optical methods. Only after it has been converted back into an electrical signal can the overhead information be read and the individual partial signals be routed independently of each other.
Much more universal is the use, tested in initial reference systems, of an optical frequency- or wavelength-division multiplex method, in which the useful information is modulated with the aid of lasers having different transmission frequencies. Consequently, it is possible using simple, passive optical filters to select the signals transmitted via an optical fiber in accordance with their frequency. With the aid of such a wavelength-division multiplex (WDM) network, it is possible for the individual signals to be transmitted transparently with regard to their coding. Therefore, many signals differently encoded in different terminal equipment facilities (TE) can be transferred simultaneously via a transmission link, as long as they differ with respect to the wavelength used.
Not yet implemented satisfactorily for WDM networks is the overhead channel for the transmission of signaling and control information. When optically transmitting WDM signals, it is not possible according to the present state of the art for the overhead information, for example using PCM or SDH techniques, to be transmitted together with the useful information. The reason this fails is that—in contrast to the fixed frame structure of such electronic transmission systems - any encoding methods at all are permitted in the WDM network for the useful signal. Another aspect, however, is the absence of powerful digital components for optical signal processing. Consequently, consideration is presently being given to switching a separate overhead channel with its own carrier frequency through the WDM network. What is problematic about this approach is that the overhead information is transmitted independently of the useful information. This means that it is just as difficult to detect frequency-selective faults as it is to detect the incorrect routing of a useful signal. Furthermore, passive optical coupling elements pose the potential problem of superposition of two overhead signals. The technical outlay required to solve this problem bears no relationship to the low cost of such elements.
The present invention relates to networks used for the optical, fiber-bound transfer of digitized information. Characteristic of a wavelength-division multiplex (WDM) network is the fact that useful information can be transmitted in the form of signals, independently of each other, via one fiber, by making use of different optical wavelengths. In this context, purely optical methods based on the different wavelengths of the signals can be used to route the useful information in network elements in different ways.
In the technical implementation of a transmission network, it is necessary that not only the useful information be transmitted via the medium, but the signaling and control information, referred to as “overhead”, as well. For this, one knows of several possible approaches for use in an optical WDM network. These are elucidated briefly in the following:
From Chawki, M J, Tholey, V Le Gac I: Management protocol of a reconfigurable WDW ring network using SDH overhead bytes; OFC 96 Technical Digest FD3, it is known to reserve additional transmission capacity for the signaling and control information when encoding the useful information. The optical WDM network is able to access this reserved bandwidth in that the useful signal is decoded and the signaling and control information is added or read out. However, this method requires a substantial technical outlay. Each accessing of the overhead information requires a conversion between the optical and electrical encoding of the complete useful signal. If the optical WDM network is used by different kinds of terminals (TE) having different encoding variants, then a corresponding number of different techniques for accessing the overhead information must be implemented in parallel.
The Draft Recommendation ITU-T G.MCS Geneva June, 1996 proposes that the signaling and control information be transmitted in a separate control channel, via the same or a different medium. S. Johansson: Transport Network Involving a Reconfigurable WDM Layer, Journal of Lightwave Technology 14 (1996) 6, pp.].:1341-1349 proposes that the signaling and control information be transmitted using so-called pilot tones, in an unused frequency range of the fiber. However, there are some desirable functionalities that cannot be implemented with either of these methods. For example, the transmission quality for the basic channel cannot be inferred from the transmission quality of the overhead information. In addition, not all configuration errors, such as mixing-up of channels, are able to be detected through the separate transfer of the control and signaling information.